1. Field of Invention
This invention is in the field of localization of resistive heat sources (hot spots), buried inside of electronic devices. Using the described invention, the positions of hot spots can be located and pinpointed in all three dimensions by analyzing the thermal heat propagation through defect-covering layers utilizing lock in amplified thermal imaging. The invention in particular enables a non destructive 3D localization of thermal active structures or defects in a vertical stack of integrated circuits and interconnect layers within system in package devices (SiP).
2. Related Art
Failure analysis of modern complex system in package devices with a three-dimensional architecture is an increasing challenge. In such devices, several integrated circuit dies are vertically stacked and interconnected by wire bond or through silicon via (TSV) technologies. The access to and analysis of internal electronic structures or interconnects is limited to non destructive techniques, like magnetic microscopy, time domain reflectometry which can be used for signal tracking. Both methods are limited in resolution and the three dimensional positioning of electrical defects are very limited or extensive. Standard failure localization methods like OBIRCH or Emission Microscopy can mostly not be applied since the optical access to buried electronic structures within the 3D architecture is limited by opaque material layers. This is most critical in case of 3D system in package devices due to the fact that thermal active structures and defects can be buried at deeper dies or interconnect levels covered by opaque die attach, redistribution or encapsulation layers. Separation of individual dies for separate failure analysis may be an option to localize defects but this process is very time consuming and can come with a high risk of creating additional preparation-related defects reducing significantly the failure analysis success rate.
Current localization techniques of thermal defects which can be electrically stimulated include the following:
Lock In Thermography (LIT)
LIT refers to a non destructive technique that detect very small temperature variations across a sample by direct thermal imaging, using an IR sensitive camera combined with pixel-wise two channel lock in correlation. LIT provides μm spatial resolution and μK sensitivity to locate thermal active structures and defects in x and y coordinates, but until now doesn't enable depth localization.
Depth of Hot Spots within or Under a Material Layer with Homogeneous Thermal Properties
For a given thermal homogeneous material there is an approximately linear dependence between the depth of a heat source underneath the surface and the time it takes the heat to reach the surface. This thermal time delay is proportional to the phase signal and can be measured by two channel lock-in thermography, which allows a recalculation of the heat source distance to the surface and therefore its depth within or under the material layer.
Thermal Scope
Conventional time-resolved temperature measurements allow to detect/display the thermal response of a sample with microsecond (μs) and milli-Kelvin (mK) accuracy.
Thermal Pulse Absorption Analysis
Based on pulsing heat onto a surface (backside or frontside) and measuring the temperature distribution/spreading in a time-resolved way. The Pulse Phase Thermography can detect sub-surface delaminations, material impurities, voids, etc. It can be used for contactless detection of weak welding joints (see, e.g., Thermosensorik, of Germany). In comparison to LIT, thermal pulse absorption is less sensitive and provides less spatial resolution.